Polls have consistently suggested Ms Rousseff would win a second round by a wide margin, but analysts say her position would be strengthened if she could win outright on Sunday.

Brazil, one of the world's most populous democracies, is also choosing local and national representatives.

Slipping lead

Maria Silveira, a Rousseff voter in Mr Lula's constituency, Sao Bernardo do Campo, outside Sao Paulo, told the Associated Press news agency: "It only makes sense to vote for the candidate who I know will continue what he started."

Brazil: Key Facts

Economy: Set for some 7.5% growth this year

Resources: Top exporter of sugar, poultry and beef; major producer of iron ore

Ms Rousseff, of the ruling Workers' Party, saw her lead in the opinion polls slip in the final days of campaigning after corruption allegations surfaced involving a former aide.

But her campaign has been boosted by energetic support from Mr Lula.

"I'm convinced the majority of people want continuity from the government," Mr Lula told a rally on Friday.

"That's why I think Dilma will win."

Ms Rousseff, 62, served as Mr Lula's chief of staff from 2005 until this year, and is a career civil servant. Her tilt at the presidency is her first attempt at elected office.

During the 1960s and 1970s she was involved in the armed struggle against Brazil's military rulers, and was jailed for three years.

The 68-year-old Mr Serra is hugely experienced, having served as Sao Paulo mayor, Sao Paulo state governor and health minister under President Fernando Henrique Cardoso Mr Lula's predecessor. He lost the presidential election in a run-off to Mr Lula in 2002.

Two other candidates for the presidency are trailing far behind in the polls.

Marina Silva of the Green Party and Plinio de Arruda Sampaio of the Socialism and Freedom Party are not expected to trouble the frontrunners.